1. Field of Invention
The present invention relates to a light emitting device power supply circuit with dimming function and a control circuit thereof. Particularly, it relates to such a light emitting device power supply circuit which conducts a bleeder current according to the conductive phase of an AC dimming voltage to avoid flicker of a light emitting device circuit, and a control circuit thereof.
2. Description of Related Art
FIG. 1A shows a schematic diagram of a prior art light emitting diode (LED) power supply circuit 100. As shown in FIG. 1A, the LED power supply circuit 100 includes a tri-electrode AC switch (TRIAC) dimming circuit 12, a rectifier circuit 14, and an LED driver circuit 16. The TRIAC dimming circuit 12 receives an AC voltage. When the AC voltage exceeds a predetermined trigger phase, the TRIAC dimming circuit 12 fires (starts-up) and is turned ON. FIG. 1B shows a schematic diagram of waveforms of the AC voltage and an AC dimming voltage generated by the TRIAC dimming circuit 12. The AC voltage is shown by a dashed line, and the AC dimming voltage generated by the TRIAC dimming circuit 12 is shown by a solid line. The rectifier circuit 14 receives the AC dimming voltage, and rectifies it to generate an input voltage Vin and an input current Iin which are inputted to the LED driver circuit 16. The LED driver circuit 16 converts the input voltage Vin to an output voltage Vout, and provides an output current to the LED circuit 11, for driving the LED circuit 11 and adjusting its brightness. In the aforementioned circuit, the TRIAC dimming circuit 12 is provided for determining a trigger phase of the AC dimming voltage to adjust an average brightness of the LED circuit 11. The LED driver circuit 16 includes a power stage circuit which has at least one power switch. The power stage circuit may be a synchronous or asynchronous buck, boost, inverting, buck-boost, inverting-boost, or flyback power stage circuit as shown in FIGS. 2A-2K.
One of the problems that the aforementioned prior art faces is that the TRIAC dimming circuit 12 includes a TRIAC device; the TRIAC device requires a large latching current to fire (start-up), but after the LED circuit 11 is turned ON, a holding current flowing through the TRIAC device is relatively lower. If what the power supply drives is a high power consuming load circuit, such as a conventional incandescent lamp, the latching current for the TRIAC device is sufficient because the conventional incandescent lamp consumes high current. However if what the power supply drives is a low power consuming load circuit, such as the LED circuit 11, the latching current for the TRIAC device is insufficient because of the low current consumption of the LED circuit 11. If the power supply circuit does not generate a sufficient latching current to fire the TRIAC device, a so-called “misfire” occurs and the LED circuit 11 will flicker perceptibly. FIG. 1C shows the waveforms of the AC voltage and the AC dimming voltage when the misfire condition occurs. On the other hand, even though the latching current is sufficient to fire the TRIAC device, a misfire may still occur if the output current Iout is too low and a current flowing through the TRIAC dimming circuit 12 is lower than the holding current, which may happen when the trigger phase is too late.
FIGS. 3A-3C show schematic diagrams of another prior art LED power supply circuit 110, which intends to solve the misfire problem of the aforementioned prior art. Different from the prior art LED power supply circuit 100 shown in FIG. 1A, the prior art LED power supply circuit 110 as shown in FIG. 3A further includes a bleeder circuit 18 in additional to the TRIAC dimmer circuit 12, the rectifier circuit 14, and the LED driver circuit 16. The bleeder circuit 18 is coupled between the rectifier circuit 14 and the LED driver circuit 16, for generating a sufficient latching current periodically to trigger the TRIAC device in the TRIAC dimmer circuit 12. After the TRIAC device is triggered, the latching current generated by the bleeder circuit 18 is consumed by a loop connected to ground. FIG. 3B shows a specific embodiment of the LED power supply circuit 110 including the bleeder circuit 18.
More specifically, the bleeder circuit 18 includes resistors R1 and R2, which are connected in series between two output nodes of the rectifier circuit 14. A divided voltage across the resistor R2 turns ON a switch Q1, which generates the latching current for the TRIAC device. A resistor R3 and Zener diodes ZD1 and ZD2 are connected in series; after the switch Q1 is turned ON, a divided voltage at the node between the resistor R3 and the Zener diode ZD1 turns ON the switch Q2, such that a holding current is generated and flows through a resistor R4. The waveforms of the AC voltage and the AC dimming voltage are shown in FIG. 3C.
Even though the prior art LED power supply circuit 110 shown in FIGS. 3A and 3B mitigates the LED flicker issue caused by the misfire of the TRIAC device, this prior art has a drawback that the TRIAC device in the TRIAC dimmer circuit 12 can not be triggered in all period. More specifically, for the bleeder circuit 18 to generate the latching current to trigger the TRIAC device, the rectified dimming signal generated by the rectifier circuit 14 at the rectified node VD must be higher than a certain level such that the divided voltage across the resistor R2 is higher than the threshold voltage of the switch Q1. If a user intends to turn low the brightness of the LED circuit 11 to an extent that the rectified dimming voltage is too low, i.e., if the conductive phase of the rectified dimming signal in FIG. 3C is too short such that the trigger phase is too close to the end of the period of the phase-cut AC dimming signal (referring to FIG. 3C), the divided voltage across the resistor R2 will be lower than the threshold voltage of the switch Q1, and the TRIAC device can not be triggered because no latching current is generated. In other words, in this prior art which uses the bleeder circuit 18, a user can not use the TRIAC dimmer circuit 12 to adjust the brightness of the LED circuit 11 in full range (the TRIAC device in the TRIAC dimmer circuit 12 can not be triggered in all period), and there is a limit to the latest timing of the trigger phase. Furthermore, in certain applications it is not necessary to provide the dimming function and therefore the TRIAC dimmer circuit 12 is not required, but in this prior art, even though there is no TRIAC dimmer circuit 12, the bleeder circuit 18 still generates current and consumes power which is completely wasted.
U.S. patent application US 2012/0319621 A1 provides a TRIAC dimming system, which includes a driver circuit having a flyback power stage circuit. This TRIAC dimming system operates a switch by a pulse width modulation (PWM) signal according to a level of the AC dimming voltage to control an auxiliary winding to generate the bleeder current. However, this prior art includes a switch which keeps switching, and the switching of the switch causes electromagnetic interferences an relatively higher noises.
In view of the foregoing, the present invention provides a light emitting device power supply circuit with dimming function and a control circuit thereof. Particularly, it relates to such a light emitting device power supply circuit and control circuit thereof, in which a bleeder current is generated according to the conductive phase of an AC dimming voltage to avoid flicker of a light emitting device circuit.